1. Field of the Invention
This invention relates to a pin transfer applicator assembly for depositing small dots of liquid materials onto a workpiece.
2. Description of the Related Art for applying small dots of liquid materials onto a workpiece. For example, automated devices for assembling printed circuit boards may include an applicator for placing small dots of liquid solder (which may be in the form of a heavy paste) onto leads of integrated circuits or other solid state devices at locations where the leads pass through the traces or interconnects of the circuit board. Once the dots of solder are in place, the dots are subjected to heat and melted so that when cooled the solder mechanically and electrically connects the leads to the traces.
Certain automated solder dot applicators include a reservoir for holding liquid solder, and a tube or needle for directing the solder to the workpiece. If desired, the needle may be mounted on a movable arm of a programmable 3-axis robotic machine such as the 3M Otto A9 system sold by the assignee of the present invention. The machine may be programmed to move the arm to different positions in order to place small dots of solder or other materials onto multiple, pre-selected locations on the workpiece.
In the past, liquid solder has often been dispensed by automated systems by use of pressurized air which is applied to the solder reservoir in order to urge the solder to flow from the reservoir and through the elongated, slender dispensing needle. However, the small passageways of such needles may occasionally clog from particles of solder, particularly when the solder is in the form of a heavy paste or suspension. Furthermore, there is often a delay from the time that pressure is applied to the reservoir and the time that the solder is discharged from the outlet of the needle, and as a consequence such systems are not entirely satisfactory for high speed operations.
Another known type of solder dot applicator is often referred to as a pin transfer applicator and includes a number of parallel, vertical pins which depend from a movable frame. The pins are arranged in an array that correlates to multiple, spaced apart locations on the circuit board. In use, the frame is first moved toward a tray containing liquid solder, and the ends of the pins are simultaneously dipped in the solder and then retracted, causing a small dot of solder to adhere to the end of each pin. Next, the frame is moved toward the circuit board, and ends of the pins advance toward corresponding, pre-selected locations on the circuit board until simultaneous contact is established between each dot of solder and the respective location on the circuit board. Finally, the frame is moved away from the circuit board, whereupon the dots of solder are transferred from the pins to the circuit board.
However, the aforementioned pin transfer applicator is somewhat unsatisfactory in that the configuration of the pin array is fixed and must be manually changed to accommodate different types of circuit boards. In addition, such a device is not entirely suitable for some high speed assembly operations because of the time necessary to move the frame supporting the pins between the tray and the circuit board.